Wellbores are formed in subterranean formations for various purposes including, for example, the extraction of oil and gas from a subterranean formation and the extraction of geothermal heat from a subterranean formation. A wellbore may be formed in a subterranean formation using a drill bit, such as, an earth-boring rotary drill bit. Different types of earth-boring rotary drill bits are known in the art, including, for example, fixed-cutter bits (which are often referred to in the art as “drag” bits), rolling-cutter bits (which are often referred to in the art as “rock” bits), impregnated bits (impregnated with diamonds or other superabrasive particles), and hybrid bits (which may include, for example, both fixed cutters and rolling cutters).
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. The drill string may comprise a series of elongated tubular segments connected end-to-end that extends into the wellbore from the surface of the formation. When weight is applied to the drill string and consequently to the drill bit, the rotating bit engages the formation and proceeds to form a wellbore. The weight used to push the drill bit into and against the formation is often referred to as the “weight-on-bit” (WOB). As the drill bit rotates, the cutters or abrasive structures thereof cut, crush, shear, and/or abrade away the formation material to form the wellbore. A diameter of the wellbore formed by the drill bit may be defined by the cutting structures disposed at the largest outer diameter of the drill bit.
Different types of bits work more efficiently against formations having different hardnesses. For example, bits containing inserts that are designed to shear the formation, such as fixed-cutter bits, frequently drill formations that range from soft to medium hard. These inserts often have polycrystalline diamond compacts (PDCs) as their cutting faces.
Roller cone bits are efficient and effective for drilling through formation materials that are of medium to high hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action, in which the inserts of the rotating cones are impacted against the formation material. This action compresses the material beyond its compressive strength and allows the bit to cut through the formation.
For still harder formation materials, the mechanism commonly used for drilling changes from shearing to abrasion. For abrasive drilling, bits having fixed, abrasive elements are preferred, such as diamond-impregnated bits. While bits having abrasive polycrystalline diamond cutting elements are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations. For these types of formations, cutting structures that comprise particulate diamond, or diamond grit, impregnated in a supporting matrix are generally more effective.
For each type of earth-boring rotary drill bit, there are virtually innumerable designs that may be employed. For example, for a fixed-cutter earth-boring rotary drill bit, a given design involves a combination of many design parameters relating to variables such as bit diameter, size and number of blades, size and number of cutting elements, location and orientation of the cutting elements, number, location, and orientation of fluid nozzles, and presence of other features such as wear knots, depth-of-cut control features, etc.
Changing one variable may lead to improvement of one characteristic of the performance of the drill bit embodying the design, but may lead to a decline in another characteristic of the performance of the drill bit. For example, increasing the aggressiveness of the cutting elements (through their number, size, location, and orientation) may lead to increased rate of penetration, but may also reduce the durability of the cutting elements and reduce the useful life of the drill bit. Thus, selection of the design of a drill bit for a given application is a highly complex process involving a balancing of the various design parameters and their advantages and disadvantages for the various performance characteristics of the drill bit.